One of the major problems in surgical reconstruction of disease-damaged calcified tissues is the attachment of prosthetic or restorative materials to the tissue surface. This is a particular problem in the restoration of teeth since the functional mechanical, thermal and environmental stresses on the restoration interface usually lead to interfacial leakage between the restoration and the tooth when the restorative material is held in place merely by gross mechanical interlocking. This leakage may lead to penetration of bacteria and/or bacterial by-products along the tissue-material interface and a renewed process of dental decay. The concept of an adhesively-bonded prosthetic and restorative material has been extensively investigated in recent years but a reliable long term bond has been found to be difficult to establish under practical clinical conditions. Further, the use of chemically-active materials to develop chemical bonding at the adhesive interface may pose biocompatibility problems. The development of an attachment mechanism which would allow functional stress transfer across the interface between reconstructive materials and calcified tissues and also minimize leakage in situations such as tooth restoration, therefore, would be extremely beneficial in many medical and dental procedures.
In dentistry a widely used mechanism for attachment of polymerizable resin materials is the so-called acid etch technique. In this procedure, the tooth enamel surface is treated for about one minute with an acid solution or gel, usually 30 to 50% phosphoric acid. This treatment results in dissolution of the outer layer of the enamel to a depth of around 20 micrometers and the production of a clean etched porous surface. When a fluid polymerizable resin composition is placed on the etched surface, capillary action draws the monomers into the surface porosity to a depth of as much as 100 micrometers. After setting of the resin composition by chemical activation or radiation-induced polymerization, a strong bond of the resin to the tooth is induced by virtue of the extensive micromechanical interlocking created through penetration of the resin into the many pores in the etched surface of the enamel. Such a bond is as strong as the weaker of the resin and the enamel surface. This acid-etch technique has been used to attach resin coatings to teeth to seal the fissures in the biting surfaces to prevent decay and to provide facings which improve the aesthetic appearance of the teeth. Another preventive dentistry application of the acid-etch technique is to bond orthodontic attachments directly to the tooth in order to remove teeth within the jaws. For restorative purposes, acid-etching is used to improve the bond of restorative resins to fractured teeth and to minimize leakage around composite resin fillings. Still other applications include the splinting of loose teeth and temporary tooth replacement.
Although the acid-etch technique has proved to be extremely useful, certain disadvantages have become apparent. Among these are the loss of a significant depth of the outer enamel which contains most of the anticariogenic fluoride and the difficulty of removing bonded orthodontic attachments because of the deep penetration of resin into the etched enamel surface. In the latter application, the clinical removal of the attached resin consumes considerable time and can result in surface damage to the tooth surface because of the instrumentation needed to remove the assimilated resin.